rtnetlink: speedup rtnl_dump_ifinfo()

When handling large number of netdevice, rtnl_dump_ifinfo()
is very slow because it has O(N^2) complexity.

Instead of scanning one single list, we can use the 256 sub lists
of the dev_index hash table.

This considerably speedups "ip link" operations
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/net_namespace.h

@@ -28,6 +28,10 @@ struct ctl_table_header;
 struct net_generic;
 struct sock;
 
+
+#define NETDEV_HASHBITS    8
+#define NETDEV_HASHENTRIES (1 << NETDEV_HASHBITS)
+
 struct net {
 	atomic_t		count;		/* To decided when the network
 						 *  namespace should be freed.

net/core/dev.c

@@ -193,18 +193,15 @@ static struct list_head ptype_all __read_mostly;	/* Taps */
 DEFINE_RWLOCK(dev_base_lock);
 EXPORT_SYMBOL(dev_base_lock);
 
-#define NETDEV_HASHBITS	8
-#define NETDEV_HASHENTRIES (1 << NETDEV_HASHBITS)
-
 static inline struct hlist_head *dev_name_hash(struct net *net, const char *name)
 {
 	unsigned hash = full_name_hash(name, strnlen(name, IFNAMSIZ));
-	return &net->dev_name_head[hash & ((1 << NETDEV_HASHBITS) - 1)];
+	return &net->dev_name_head[hash & (NETDEV_HASHENTRIES - 1)];
 }
 
 static inline struct hlist_head *dev_index_hash(struct net *net, int ifindex)
 {
-	return &net->dev_index_head[ifindex & ((1 << NETDEV_HASHBITS) - 1)];
+	return &net->dev_index_head[ifindex & (NETDEV_HASHENTRIES - 1)];
 }
 
 /* Device list insertion */

net/core/rtnetlink.c

@@ -682,22 +682,33 @@ static int rtnl_fill_ifinfo(struct sk_buff *skb, struct net_device *dev,
 static int rtnl_dump_ifinfo(struct sk_buff *skb, struct netlink_callback *cb)
 {
 	struct net *net = sock_net(skb->sk);
-	int idx;
-	int s_idx = cb->args[0];
+	int h, s_h;
+	int idx = 0, s_idx;
 	struct net_device *dev;
+	struct hlist_head *head;
+	struct hlist_node *node;
+
+	s_h = cb->args[0];
+	s_idx = cb->args[1];
 
+	for (h = s_h; h < NETDEV_HASHENTRIES; h++, s_idx = 0) {
 		idx = 0;
-	for_each_netdev(net, dev) {
+		head = &net->dev_index_head[h];
+		hlist_for_each_entry(dev, node, head, index_hlist) {
 			if (idx < s_idx)
 				goto cont;
 			if (rtnl_fill_ifinfo(skb, dev, RTM_NEWLINK,
 					     NETLINK_CB(cb->skb).pid,
-				     cb->nlh->nlmsg_seq, 0, NLM_F_MULTI) <= 0)
-			break;
+					     cb->nlh->nlmsg_seq, 0,
+					     NLM_F_MULTI) <= 0)
+				goto out;
 cont:
 			idx++;
 		}
-	cb->args[0] = idx;
+	}
+out:
+	cb->args[1] = idx;
+	cb->args[0] = h;
 
 	return skb->len;
 }